The present invention relates generally to an apparatus and method for spot welding overlapping coated sheets together.
Zinc coated sheet steel is commonly used in the automobile industry. Spot welding is often employed to weld overlapping sheets of zinc coated steel together at a desired point. A problem with spot welding zinc coated steel is that the outgassing of zinc on the surface of the steel during welding can cause a violent explosion of material from the weldpool, causing damage to the welding apparatus.
Additionally, if the gap between the overlapping sheets is small, the gases cannot escape and can pressurize due to heating. The pressurized vapor can cause localized thinning of the top sheet around the periphery of the weld, decreasing the weld strength. This thinning problem greatly increases by the presence of oils or waxes on the surfaces of the sheets.
In a prior welding apparatus, oxygen has been employed to shield the zinc vapors released during welding. The oxygen increases the fluidity and wettability of the melted metal, allowing the zinc vapors to bubble out and escape through the molten metal without explosion.
Aluminum alloy has also been used in the automotive industry as a material for vehicle bodies due to its high strength and low weight. A surface film oxide forms rapidly on aluminum and its alloys when exposed to the atmosphere, preventing fusion between the sheets. Chemical cleaning process have been utilized to reduce the hydrated oxide film and to remove oils and lubricants. However, the oxide removal processes are time consuming, expensive and often employs chemicals which are toxic, hazardous, or environmentally unfriendly.
As the high thermal conductivity of aluminum alloy ranges from three to five times that of steel, significantly higher inputs of energy are also required for fusions, creating difficulties in the welding of aluminum alloy.
A welding apparatus spot welds two or more overlapping sheets having a coating. The overlapping sheets can be zinc coated steel, aluminum alloy with an oxide coating, or other coated materials.
A hole is formed in the upper sheet at the desired point of welding. The sheets are positioned such that the hole is aligned with the plasma arc torch of the welding apparatus.
The plasma arc passes through the hole and contacts the upper surface of the sheet below the upper sheet. The torch heats the upper surface of the sheet, allowing entrapped vapors and lubricants to vent freely. Filler wire melted by the torch is added to fill the hole and secure the overlapping sheets together. If the overlapping sheets are zinc coated steel, preferably the filler wire is copper based and includes alloys of silicon bronze, aluminum bronze, and cupro-nickel. If the overlapping sheets are aluminum alloys with an oxide coating, the filler wire is preferably aluminum.